Polyesters and particularly polyethylene terephthalate are being employed in an increasing array of packaging applications and particularly in the field of food and beverage drink packaging. By way of example such applications include polyester coated paperboard for trays, soft drink beverage bottles, blister packs for the packaging of cold cuts and films for various food wrap applications. However, no matter the growing use of polyesters for such applications, one area in which growth has not yet occurred is that of containers for hot fill food applications. By the term "hot fill" is meant those packaging operations wherein the prepared food is added hot to the package container or where the food is first added to the container and then subjected to a subsequent heating and sterilization operation.
To date most such hot fill food applications employ mainly the familiar metal can or where it is desired that the packaged food or food product be observable containers of glass or thermoplastics such as polypropylene. In the case of the latter, containers can be prepared by means of well-known processes such as, for example, that disclosed in U.S. Pat. No. 3,499,188. In accordance with the teachings of this referenced patent, hollow articles can be provided from a flat sheet blank in a single continuous procedure.
The major drawbacks in using polyesters in such hot fill food application are basically two-fold. First, biaxially oriented polyester containers undergo considerable shrinkage when heated to temperatures at or near their glass transition temperature and second, such containers undergo a change in crystallinity at elevated temperatures such as those encountered in hot fill food applications resulting in their becoming opaque. The present invention provides a process whereby containers for hot fill applications can be prepared from polyesters and particularly polyethylene terephthalate, which exhibit a reduced tendency to shrink, retain their good clarity and impact strength and in addition are of relatively uniform wall and bottom thickness.